If you and your significant other were the last two people on Facebook, it’d probably look something like Pair.

The app, which launched this week for the iPhone, lets you communicate with one other person–ideally a spouse or romantic partner. Upon signing up, you provide an e-mail address for the person you want to pair with, and that person gets an invitation to the service. You can unpair whenever you want, but you can’t pair with more than one person at a time.

Pair is not a social network, but it behaves like one. The main section of the app is a single stream of communication between you and your loved one, with pictures, video and shared locations posted directly on the timeline. When both partners are online, the app becomes like a chat program, letting you know when…